1. Field of the Invention
This invention relates in general to electronic devices and, more particularly, to improved means and methods for providing leads to semiconductor or other electronic chips. As used herein the words device or devices are intended to refer to all kinds of electronic devices and integrated circuits which use connection means and leads of the sort described herein, including but not limited to semiconductor based devices.
2. Background Art
In the electronics art, particularly the semiconductor device and circuit art, it is commonplace to provide light weight leads to devices that primarily function for signal processing and heavy leads to devices that carry substantial current. Wire bonding of thin wires and tab bonding of metal foils are examples of techniques commonly used for signal processing devices. Such wires or foils generally carry only microamperes or milliamperes, are typically a mil to a few mils thick, and are generally welded directly to the bonding pads on the device.
With higher current devices, such as power diodes and transistors or integrated circuits where currents of amperes to tens or hundreds of amperes are required, the leads bonded to the device must be more robust. It is commonplace to use metal leads that are of the order of ten to hundreds of mils thick. They are frequently attached by soldering to the bonding areas on the device.
When the device to be connected has an irregular surface, it becomes more difficult to attach such heavy leads without creating failure modes which can reduce manufacturing yield and reliability. For example, when power semiconductor die are manufactured using masking and/or passivating dielectrics the bonding pads may be partially surrounded by a raised dielectric whose outer surface is higher than the bonding pad surface. When the usual prior art flat leads are soldered to such a bonding pad, the solder volume and spreading is difficult to control. Solder runout on the dielectric surface becomes more likely and bridging of the solder around the edge of the chip may cause electrical shorts that render the device inoperative or unreliable. Thus, a problem continues to exist in bonding heavy leads to power devices where the bonding pad or other connection point is recessed below the adjacent outer surface, for example, where the bonding pad is at least partially surrounded by a dielectric or other material not intended to contact the lead and of greater elevation relative to the substrate surface than the bonding pad.
Accordingly, it is an object of the present invention to provide an improved means and method for bonding leads to devices having a bonding pad or contact region which is depressed relative to at least part of the outer surface of the surrounding region cf the device.
It is a further object of the present invention to provide an improved means and method for bonding leads to semiconductor devices, as above, and which also can carry continuous currents of about an ampere or more.
It is an additional object of the present invention to provide an improved means and method for bonding leads to devices, as above, and which are desired to be attached by use of solders.
It is a further object of the present invention to provide an improved means and method for bonding leads to devices, as above, while maintaining close die-lead spacing and solder filling of the bonding pad and lead-bonding pad space, so as to enhance the surge capability of the device.
As used herein the words solder or solders are intended to include any electrically conductive attachment material which is semi-solid or at least partially liquid at some time during the lead attachment process. Nonlimiting examples are conventional metal or metal alloys or metal loaded epoxies or other conductive plastics or glasses, and the like.